A number of devices are designed to monitor the level of liquids in relatively motionless chambers. Tubular sight glasses have been mounted on the outside of chambers wherein the liquid rises inside the glass to indicate the internal level. Other devices utilize various types of floats that rise or fall according to the liquid level. None of these devices, however, have proved satisfactory for monitoring liquid levels in a chamber subject to motion. Moreover, these devices cannot monitor the condition or quality of the liquid.
In some devices, a light beam is projected across a gap onto a photoelectric sensor which can measure the diminution of light caused by the presence in the gap of a liquid, or the presence of contaminants in gases. Yet other devices utilize spectral absorption properties of particular molecules to identify the presence or absence of those molecules.
Commonly, oil level indicators use thermistors whose conductivity depends on temperature. Resistance within the thermistor increases as oil covers the element and decreases its temperature by conducting heat away from it. The temperature of the thermistor rises, and therefore its resistance declines, as the oil recedes from the element.